1. Field of the Invention
The present invention relates to a scanning optical system, an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Optical scanning devices are widely known with regard to a digital copier, a laser printer, a laser plotter, a laser plate-making apparatus and so forth.
A scanning optical system is used in an optical scanning device, and is an optical system condensing a beam deflected by an optical deflector and forming a beam spot for scanning on a surface to be scanned. The beam spot thus formed on the surface to be scanned scans the surface to be scanned and performs image writing.
Recently, it is demanded for such an optical scanning device to perform scanning in high density at high speed. Further, in many cases, by the reason of layout of an image forming apparatus employing such an optical scanning device, ‘a long back length’ is demanded for the scanning optical system.
‘The back length’ is a distance from one of imaging elements (such as lenses, concave mirrors) included in the scanning optical system which one is disposed closest to a surface to be scanned, to the surface to be scanned.
In an image forming apparatus employing an optical scanning device, it is difficult to change a structural configuration.
For example, in the image forming apparatus, a photoconductive photosensitive body is used as a photosensitive medium, it is charged uniformly by a charging unit, then, an image is written thereon by an optical scanning device, the thus-formed electrostatic latent image is developed into a toner image by a developing device, the toner image is transferred onto a transfer paper and is fixed thereonto, and thus, image forming is performed.
In such a case, structural positional relationship of the mechanical devices needed for the image forming process such as the charging unit, developing device, transfer device and fixing device, with respect to the photosensitive body, is limited due to the mechanical configurations thereof. Accordingly, the positions thereof are almost fixed, and thus, it is difficult to change these positions.
In contrast to this, with regard to the optical systems of the optical scanning device, an optical arrangement from a light source to a surface to be scanned (substantially, the photosensitive surface of the photosensitive body) can be changed freely to some extent, and can be optically designed according to a layout of the charging unit, developing device and so forth, in the image forming apparatus. Therefore, there are many cases where ‘the scanning optical system having a long back length’ is demanded in a view point of layout.
Recently, ‘the scanning density’ of the optical scanning device is demanded to be higher, for example, 1200 dpi (the number of dots per inch) or 2400 dpi. In order to achieve high density of scanning, it is necessary to reduce a diameter of a beam spot formed on the surface to be scanned.
In order to reduce the diameter of the beam spot, it is necessary to enlarge the diameter of the beam incident on the scanning optical system. When the diameter of the beam incident on the scanning optical system increases, it is important to make uniform wave-optic wavefront aberration whatever the image height is, as well as to improve the geometric-optic characteristics such as curvature of field and imaging magnification, for the scanning optical system.
As a system for increasing the scanning speed of the optical scanning device, ‘a multi-beam system’ in that the surface to be scanned is scanned by a plurality of beam spots at the same time is watched. Therein, a light source in an LD array system in that light emitting sources of monolithic semiconductor laser array are arranged in row, or in a beam combining system in that beams from a plurality of semiconductor laser are combined, is used.
When such a light source in the LD array system or beam combining system is used, similarly to the case of the scanning optical system in the single beam system, it is possible to use in common the optical systems on the light path from the light source to the surface to be scanned for a plurality of beams. Accordingly, it is possible to achieve the optical system in the multi-beam system with high stability against structural change.
Also in the case of the multi-beam scanning system, it is necessary to reduce the diameter of each beam spot formed on the surface to be scanned for achieving high-density scanning. For this purpose, it is necessary to keep the wave-optic wavefront aberration smaller as in the case of single-beam system.